A pacemaker is an implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia—a condition in which a patient cannot maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, which was typically set at a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, and in more advanced devices, could be set remotely after implantation.
Early advances in pacemaker technology included the ability to sense a patient's intrinsic cardiac activity (i.e., the intercardiac electrogram, or “IEGM”). This led to the development of “demand pacemakers,” so named because these devices deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective, cardiac contraction which occurs within a predetermined time period (commonly referred to as the “escape interval”) following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, a demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable, because it extends battery life.
Pacemakers such as those described above proved to be extremely beneficial in that they successfully reduced or eliminated seriously debilitating and potentially lethal effects of bradycardia in many patients. However, the early devices were not adjustable “in the field”—that is, the heart rates maintained by these devices were not adjustable in accordance with changing levels of physical exertion. Thus, during periods of elevated physical activity, some patients were subject to adverse physiological consequences, including light-headedness and episodes of fainting, because their heart rates were forced by the pacemaker to remain constant at an inappropriately low rate. Also, some patients were subject to discomfort resulting from heart rates that were maintained-higher than would normally be appropriate during periods of rest.
A major advance in pacemaker technology was the development of “rate-responsive pacemakers.” These devices are capable of adjusting the patient's heart rate in accordance with metabolic demands, even as those demands vary as a result of changing levels of physical exertion. Rate-responsive pacemakers typically maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increase the maintained heart rate in accordance with increased levels of physical activity until a maximum rate is reached. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and the slope or curve between the minimum heart rate and the maximum heart rate are programmable, so that they may be configured to meet the needs of a particular patient.
In order to provide rate-responsive pacing therapy, a pacemaker must be capable of correlating an indicator of physical activity to an appropriate heart rate. The generally accepted technique for providing rate-responsive pacing is to employ sensors that transduce mechanical forces associated with physical activity. A widely used sensor of this type incorporates a piezoelectric crystal which generates a measurable electrical potential when a mechanical stress resulting from physical activity is applied to the sensor. U.S. Pat. No. 4,140,132 to Dahl and U.S. Pat. No. 4,428,378 to Anderson et al., which are incorporated herein by reference, describe examples of rate-responsive pacemakers that maintain a patient's heart rate in accordance with physical activity as measured by a piezoelectric sensor incorporating a piezoelectric crystal. Besides piezoelectric crystals, other piezoelectric activity sensors employed in pacemakers use a cantilever beam having a film of a piezoelectric polymer adhered to a surface of the beam. Examples of such cantilever type passive sensor are described in U.S. Pat. No. 5,833,713 to Moberg and U.S. Pat. No. 5,383,473 to Moberg, which are incorporated herein by reference.
Piezoelectric activity sensors are passive devices, meaning they do not require an external excitation current or voltage to operate. Rather, they convert mechanical motion into a detectable electrical signal, such as a back electro magnetic field (BEMF) current or voltage. Accordingly, since minimizing current drain and power consumption is critical with battery powered implantable devices, piezoelectric activity sensors are the primary type of activity sensor used in rate-responsive pacemakers. Nevertheless, despite their widespread use, piezoelectric activity sensors have certain limitations. For example, a typical passive piezoelectric activity sensor is only able to provide one dimension of information, i.e., acceleration. While this may be enough information to provide acceptable rate responsive pacing, it would be beneficial if the dynamic and spatial range of information obtained from activity sensors could be increased.
More specifically, it would be beneficial if more complex and informative trends about a patient's physical activity, or lack thereof, can be obtained. Such information may be indicative of how often the patient is active over a specified period of time, or even how often the patient is lying down, sitting up, walking and/or running. This type of information may be very beneficial to a physician that is monitoring the progression of a disease, such as congestive heart failure (CHF). CHF is a disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body's tissues, resulting in fatigue, weakness, and inability to carry out daily tasks.
More complex multi-dimension activity sensors have been proposed. For example, U.S. Pat. No. 6,658,292 to Kroll et al. describes a three dimensional accelerometer-based position sensor that can sense a patient's movement, position and activity status, such as whether the patient is ascending/descending stairs or is sitting up from a lying position, based on the vertical velocity and minute ventilation data. For another example, U.S. Pat. No. 6,466,821 to Pianca et al. describes an AC/DC multi-axis accelerometer that can also be used to determine patient activity and body position. An example of a commercially available dual-axis accelerometer is the model ADXL210 available from Analog Devices of Norwood, Mass. While there would be informational advantages to including such multi-dimensional activity sensors in a pacemaker, such sensors typically require a current in the range of 0.1 mA or greater (in contrast, the parasitic current associated with a passive piezoelectric activity sensor is in the range of about 5 to 15 μA, i.e., 0.005 to 0.015 mA). Since minimizing current drain and power consumption is critical with battery powered implantable devices, it has not yet been practical to include such relatively high powered multi-dimensional activity sensors within pacemakers.
Chronic diseases such as CHF require close medical management to reduce morbidity and mortality. Because the disease status evolves with time, frequent physician follow-up examinations are often necessary. At follow-up, the physician may make adjustments to the drug regimen in order to optimize therapy. This conventional approach of periodic follow-up is unsatisfactory for some diseases, such as CHF, in which acute, life-threatening exacerbations can develop between physician follow-up examinations. It is well know among clinicians that if a developing exacerbation is recognized early, it can be more easily and inexpensively terminated, typically with a modest increase in oral diuretic. However, if it develops beyond the initial phase, an acute heart failure exacerbation becomes difficult to control and terminate. Hospitalization in an intensive care unit is often required. It is during an acute exacerbation of heart failure that many patients succumb to the disease.
It is often difficult for patients to subjectively recognize a developing exacerbation, despite the presence of numerous physical signs that would allow a physician to readily detect it. Furthermore, since exacerbations typically develop over hours to days, even frequently scheduled routine follow-up with a physician cannot effectively detect most developing exacerbations. It is therefore desirable to have a system that allows the routine, frequent monitoring of patients so that an exacerbation can be recognized early in its course. With the patient and/or physician thus notified by the monitoring system of the need for medical intervention, a developing exacerbation can easily and inexpensively be terminated early in its course.
Accordingly, it would be advantageous to provide implantable cardiac devices that can obtain complex and informative trends about a patient's physical activity, or lack thereof, while keeping the current drain and power consumption within an acceptable range. More generally, it is desirable to provide implantable cardiac devices that can obtain disease progression information in an energy efficient manner.